Many integrated devices incorporate circuitry to generate a regulated voltage from an external power source for internal use in the device. For example, an electrically erasable and programmable read-only memory (EEPROM) may incorporate such a circuit for internally generating a programming voltage from an external supply voltage. Or, a memory device such as a dynamic random access memory (DRAM) and other types of integrated circuits may include such circuitry for internally generating from an external supply voltage a substrate or back bias voltage to adjust the thresholds of field-effect transistors (FETs) formed in the substrate. Such internal voltage generation often reduces the number of external supply voltages that the integrated device requires for operation.
Typically, such generation and regulation circuitry includes a voltage generator, such as a charge pump, and a voltage regulator that maintains the internally generated voltage at a predetermined voltage level or within a predetermined voltage range. For example, the regulator may activate the generator when the voltage is not at the predetermined level or is outside the predetermined range, and may deactivate the generator once it drives the voltage to the predetermined level or within the predetermined range.
It is often desirable to design an integrated device such that it can operate over a relatively broad range of supply voltages. For example, it may be desirable to design a DRAM that can operate from a supply voltage ranging from approximately 3 to 6 volts.
Some existing voltage regulation circuits can maintain the regulated voltage at a substantially fixed level over the range of external supply voltages within which the integrated device is operational. Such regulation circuits, however, often require a stable reference voltage and associated comparison circuitry. The circuitry that generates the stable reference and the associated comparison circuitry often have a substantial number of components that occupy a relatively large area of the integrated device.
Other existing voltage regulation circuits have a minimum number of components, and thus occupy a minimum area of the integrated device. The level at which the regulator maintains the generated voltage, however, is often sensitive to changes in the supply voltage. That is, the level of the regulated voltage shifts as the supply voltage varies. Such shifts in the regulated voltage are often undesirable.